Ad Astra Observatory

After speaking with Art tonight about my guiding workflow, it seems that I have been doing things… in a manner which would probably give me problems if I tried to do longish exposures.

Art has been talking to me for years about how he always guides on a star that is in the same field of view as the object he’s imaging.

This has always sounded like a mixture of hubris and idiocy to me, since the ST-4 can’t seem to pick up any stars that I can’t see in the finder (about mag 6 or brighter), which means I essentially *never* guide on a star in the same FOV as the target object.

We finally hashed it out, and I realized that I made two errors that compounded upon each other.

The first error is that I have been using guide exposures of less than 0.5sec. This comes from my CloudyNights background of trying to force a webcam to do a guider’s work. Webcams have a max exposure time of 0.2sec, so a beginner imager using one is restricted to guiding on bright stars, but that’s OK because they are also probably going to be limited in their total exposure time per subframe by other things, like a beginner mount, a beginner imaging camera, etc. I don’t have a beginner’s guider, but the way I learned to use it was by talking with people who do. There are a lot of myths about astrophotography out there. I’m good at seeing most of them, but this one I missed.

Because of the short guide exposures, I was having trouble finding bright enough guidestars. That led to the second error, which is pointing the guidescope and the main scope in slightly different directions, so that the main scope is over the target, and the guidescope is over a sufficiently bright star.

For subframes less than 3 minutes or so, having the guidescope and main scope pointing in slightly different directions does not make a difference. Sure, over the course of the night, you’ll see the object drifting around the FOV, but that’s what stacking software is for, right?

Now that I’m hitting the end of that “beginner” type workflow, and asking the rig to do 5, 10, and 20 minute subframes, this method of guiding is leading to problems with orthogonality. That is, since the guidescope is now not aligned with the same polar axis as the rest of the mount, it appears to be misaligned from the NCP, and thus sends spurious guide commands to the mount.

No matter which scope I had doing the guiding, I was doing the same thing (misaligning the two scopes), so I was seeing the same type of problem, trailed stars that move in an elliptical “orbit” over the course of the night.

The answer is clear. Always point the guidescope and main scope in the same direction, period.

This means that I will have to find a guidestar that is close to the object I need to image.

And in order to do that, I will need to use longer guider exposures, on the order of 2-5sec. The longer exposures will mean that calibrating the guide images will be key, so darks (and maybe bias and flats, too) will need to be shot for the guide camera.

However, longer images should mean that I’ll be able to pick up dimmer stars, and hopefully will be able to find a star in every FOV.

I still won’t be able to go unguided. That’s a level of mount tuning that I may never reach. But if I can get to the point where I am able to shoot subframes as long as I want, and guide my way all the way through them, that will probably be enough.

By the way, with this information in mind, the OAG probably *will* “solve” the problem I’m having with “flexure”. I would have to start using longer guide exposures when the OAG forced me to find a guidestar in the same FOV as the target…

Now I await a clear night to test this latest theory.